With a demand for high speed communications in the Internet and corporate networks, use of optical fiber cables has been spreading rapidly.
The optical fiber is made of quartz glass, and accordingly is quite vulnerable to an external force and an external environment. For this reason, a protective coating layer generally coats the circumference of an optical fiber to protect the optical fiber from the external force and the external environment. The optical fiber thus coated with the protective coating layer is called a coated optical fiber. Then, an optical fiber ribbon in the form of a ribbon is formed in such a way that multiple optical fibers are arrayed and an ultraviolet curable resin coats the circumference of the coated optical fibers.
An installation of an optical fiber ribbon thus formed requires work to take out the coated optical fibers by separating the protective coating layer from the optical fiber ribbon. The separability of the protective coating layer from the optical fiber ribbon has a large influence on working efficiency in handling the optical fiber ribbon. For this reason, one of approaches having been taken so far is to evaluate an optical fiber ribbon by using single core separability as an indicator indicating how easy the protective coating layer can be separated from the coated optical fibers (for example, see Patent Document 1). The single core separability of an optical fiber ribbon is determined by an adhesion between a ribbon material and outermost layers of coated optical fibers. If the single core separability is excessively good, in other words, the adhesion is excessively weak, the ribbon material layer and the outermost layers may be separated from each other when soaked in water. Such separation may increase a transmission loss of the optical fibers. On the other hand, if the single core separability is excessively bad, in other words, the adhesion is excessively strong, an installation operation requires a long time for single core separation work, and consequently is lowered in working efficiency.
As material development for protective coating layers of optical fibers has been advancing, the material characteristics thereof have been improved. With this improvement, the demand for improvement of ribbon material characteristics has become increasingly strong. In this regard, single core separability of the optical fiber ribbon largely depends on the characteristics of a ribbon material used in the optical fiber ribbon. An ultraviolet curable type of urethane acrylate resin generally used as a ribbon material for an optical fiber ribbon contains a polyol component having a long chain part as a skeleton component. The ribbon material is composed through reaction of this polyol component with an isocyanate component, a reactive oligomer containing hydroxyacrylate having an unsaturated double bond, a reactive multifunctional and/or reactive monofunctional acrylate monomer also having an unsaturated double bond, a vinyl monomer, a photopolymerization initiator, and two or more additives. With the thus composed ribbon material irradiated with ultraviolet rays, the photopolymerization initiator generates radicals and the unsaturated double bonds of the oligomer and the monomer are cross-linked. As the polyol component of the oligomer, there can be cited polypropylene glycol, an ethylene oxide-butadiene oxide copolymer, and polytetramethylene glycol. Among them, the polytetramethylene glycol (hereinafter, abbreviated as “PTMG”) has high thermal resistance, and is also excellent in mechanical strength, elongation characteristics, and toughness. Thus, the PTMG is widely used as a ribbon material.